The White House sent United Nations Ambassador Susan Rice to Capitol Hill on Tuesday to gauge the response of Senate Republicans to her potential nomination to replace Secretary of State Hillary Clinton. She met behind closed doors with Sens. John McCain (R-Ariz.) and Kelly Ayotte (R-N.H.) and Lindsey Graham (R-S.C), all of whom have been critical of her over comments she made following the Sept. 11 attacks in Libya.

Rice, who was accompanied by acting CIA Director Michael Morell, tried to explain to the senators why she blamed the attacks that killed Ambassador Christopher Stevens and three others on an amateur video posted on YouTube.

The meeting lasted approximately 90 minutes but seemed to elicit more questions than answers.

"I am more disturbed now than before," said Graham.

"I'm significantly troubled by the answers we got and didn't get," McCain told Fox News. "I'm not going to go into the whole tick tock, but it's clear what my concerns are. She told the American people things that were patently untrue ... As I said at the time people don't go to demonstrations with mortars and rocket propelled grenades."

Former CIA Director David Petraeus testified before Congress two weeks ago that the agency knew from the beginning that the attacks were premeditated and organized by terrorists.

The White House has hinted for weeks that Rice may be a top-tier candidate to replace Clinton, who is expected to leave her post as soon as a suitable replacement is found. When White House spokesman Jay Carney was asked about the possibility that President Obama would tap her for the post, he only spoke of her qualifications.

"Ambassador Rice has done an excellent job at the United Nations and is highly qualified for any number of positions," Carney said.

McCain threatened to block her nomination if Obama tapped her as his nomination for the State Department's top post.

In a press conference a week after the November elections, the three senators took a hard-line approach toward Rice, even going as far as threatening to block her nomination.

"I will do everything in my power to block her from being the United States Secretary of State," McCain said during the Nov. 14 press conference. "She has proven that she either doesn't understand or she is not willing to accept evidence on its face."

"She's not qualified," McCain also said on CBS's "This Morning." "Anyone who goes on national television and in defiance of the facts, five days later – We're all responsible for what we say and what we do. I'm responsible to my voters. She's responsible to the Senate of the United States. We have our responsibility for advice and consent."

Later that same day, President Obama came to the defense of Rice, saying that if McCain or Graham had an issue with her then they could take it up with him instead.

"If Senator McCain and Senator Graham and others want to go after someone, they should go after me," he said. "And I'm happy to have that discussion with them…I don't think there's any debate in this country that when you have our Americans killed that's a problem," Obama said. "They won't get any debate from me on that. But when they go after the U.N. ambassador, apparently because they think she's an easy target, then they've got a problem with me."

Graham quickly answered Obama, making it clear that the three GOP senators ultimately would hold him accountable.

"Mr. President, don't think for one minute I don't hold you ultimately responsible for Benghazi. I think you failed as commander in chief before, during and after the attack," Graham said.

But some political observers think the Arizona Senator and former GOP presidential candidate may be backing away from his hard-line stance in opposing Rice's nomination.

In an interview with reporters on Monday, McCain stated he would ask Rice "the same questions I've been talking about on every talk show in America." Asked whether he thinks she's still unfit for Secretary of State and what he was hoping for, McCain interrupted and said, "I'm not hoping for anything. She asked to see me and I agreed to see her."

Rice, who is 48, graduated from Stanford University in 1986 and attended Oxford prior to working as a policy aide to former Massachusetts Gov. Michael Dukakis during his failed 1988 presidential race. In 1993 she went to work in the Clinton administration, holding various positions at the National Security Council and the State Department.

She is married to ABC News producer Ian Cameron and they have two children.